test: Network packet processing chain

[benaadams]

Problem

Magma.Network is the core processing engine - delegate chaining, packet routing, error propagation. Completely untested.

Sub-tasks

• Create test/Magma.Network.Facts project, add to solution
• Single-delegate chain: packet in, processed packet out
• Multi-delegate chain: verify ordering, short-circuit behaviour
• Error propagation: exception in a delegate, verify chain halts or continues as designed
• IPacketTransmitter / IPacketReceiver mock implementations for isolation

Dependencies

Depends on: nothing (tests existing code)
Unlocks: #108 (UDP chain integration), #131 (Kestrel transport)

Phase

Phase 0 (Foundation) - can start immediately